Rakefile doc: added section on rakefile path, added note on environment variables.

Hi!

I updated the Rakefile doc with the following:

* I added a section about where to place Rakefiles. (I've not seen this mentioned in the Rake tutorials I've read around the web.)

* I expanded the example in the "Arguments and the Environment" section and added a note on environment variable persistence. 

Hope it's useful. Thanks :smile: